Silicon carbide (SiC) ceramics has excellent properties such as environment resistance, heat resistance, abrasion resistance, high rigidity, high heat conductance, low thermal expansion, and the like and, therefore, is used as a high-temperature structural member, an abrasion resistant member and the like. Especially, the properties of the SiC ceramics are utilized to practically use the SiC ceramics for jigs for semiconductor manufacturing devices and other semiconductor-associated parts in these years. Besides, it is also studied for application to industrial equipment such as nuclear, gas turbine and other energy equipment, pump parts, mechanical seal parts, sliding parts and the like.
As a process for producing the SiC ceramics, a reaction-sintering method is known other than a powder sintering method using the same sintering aids as used for an ordinary ceramics material. For example, the reaction-sintering method for SiC forms a mixture of SiC powder as an aggregate with carbon powder, a resin and the like into a desired shape, keeps heating the compact to the melting temperature or more of silicon and impregnates the molten silicon. The reaction sintering of SiC is a sintering method involving a reaction between carbon and silicon.
The reaction-sintering method for SiC has a lower sintering temperature than that of the powder sintering method and can provide a dense body without using sintering aids. The reaction-sintering method does not have a large shrinkage in size at sintering in comparison with the powder sintering method and can produce a large-size part and a complex shape part having a near net shape, providing an advantage capable of largely reducing the cost required for fabrication into the final shape. Thus, the reaction-sintering method is expected as a method which can produce SiC ceramics with a high purity at a low cost.
But, the SiC ceramics (reaction-sintered SiC) produced by the reaction-sintering method is generally known that it is poor in mechanical properties such as strength, fracture toughness values and the like in comparison with the SiC sintered body produced by the powder sintering method. Therefore, the reaction-sintered SiC is not applied to parts and devices which are required to have high strength at present.
Specifically, the reaction-sintered SiC contains a liberated silicon (Si) phase derived from its production method. Though the liberated Si phase contributes to make the reaction-sintered SiC dense, it is a cause of lowering the strength of the reaction-sintered SiC because it tends to become the starting point of fracture. A conventional reaction-sintered SiC has its strength or the like heavily degraded in comparison with the SiC sintered body produced by the powder sintering method because it is determined to have a large content of the liberated Si phase in order to enhance denseness, productivity and the like.
It is general for the conventional reaction sintering to use SiC powder having a relatively large grain diameter as an aggregate in order to improve the denseness, productivity and the like. However, the SiC ceramics produced by the conventional reaction-sintering method has a problem that the microstructure based on the aggregate SiC and reaction product SiC tends to become nonuniform because of the aggregate SiC having a relatively large grain diameter. The nonuniform microstructure becomes a cause of degrading the strength of the reaction-sintered SiC.
It is tried to reduce the amount of the liberated Si phase present in the reaction-sintered SiC against the above-described lowering of strength because of the liberated Si phase. But, simple reduction of the content of the liberated Si phase in the reaction-sintered SiC cuts the network structure of the liberated Si phase into pieces, resulting in producing a large volume of pores. Besides, microcracks are easily produced as the volume of the SiC produced by the reaction sintering expands. The pores and microcracks present in the reaction-sintered SiC cause lowering of the strength in the same manner as in the ordinary ceramics material.
Besides, Japanese Patent Laid-Open Application No. 2001-19551 describes a production method by which silicon is prevented from remaining by immersing a compact in a liquid silicon source (alkoxysilane or the like) and sintering at a temperature in a range of 1500 to 2000° C. But, the complete removal of the liberated Si phase tends to degrade the density because the liberated silicon phase contributes to making the reaction-sintered SiC dense. The strength of the reaction-sintered SiC has not been enhanced satisfactorily because the denseness is degraded. Sintering at a relatively high temperature impairs the advantage of the reaction-sintering method capable of sintering at a low temperature.
As described above, the conventional reaction-sintering method has not reached a level that a high-strength SiC ceramics is produced with a good reproducibility. A method of producing SiC parts by the reaction-sintering method has a small shrinkage in dimensions at the time of sintering and can reduce a cost required for fabricating to provide a final shape but its application to parts having a complex shape is not satisfactory. Specifically, the compact shape is limited, and the machining cost increases because the conventional reaction-sintering method causes a volume expansion when the liberated Si phase solidifies. Besides, there is a problem that cracks and the like are easily caused at the time of solidification.
In this connection, Japanese Patent Laid-Open Application No. HEI 8-183661 describes a production method by which a temperature range of ±10° C. of at least the melting point of silicon is cooled at a cooling rate of 12° C./hr in a cooling process after the impregnation of silicon. According to this reaction-sintered SiC production method, the occurrence of cracks is suppressed to some extent, but an essential subject of providing a high strength has not been improved.
The present invention provides a silicon carbide matrix composite material which can be applied to a variety of members and parts required to have a high strength by improving the mechanical properties such as strength, toughness and the like of the reaction-sintered SiC and enhancing reliability and durability, and a process for producing it. The present invention also provides a process for producing parts of a silicon carbide matrix composite material by which parts having a complex shape and a large size of the silicon carbide matrix composite material can be produced inexpensively with their strength and toughness improved.